Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. Among other things, lithium secondary batteries having a high-energy density and voltage, a long cycle lifespan and a low self-discharge rate are commercially available and widely used.
As cathode active materials for the lithium secondary batteries, lithium-containing cobalt oxide (LiCoO2) is largely used. In addition, consideration has been made to using lithium-containing manganese oxides such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure, and lithium-containing nickel oxides (LiNiO2).
Of the aforementioned cathode active materials, LiCoO2 is currently widely used due to superior general properties including excellent cycle characteristics, but suffers from low safety, expensiveness due to finite resources of cobalt as a raw material, and limitations in practical and mass application thereof as a power source for electric vehicles (EVs) and the like.
Lithium manganese oxides, such as LiMnO2 and LiMn2O4, are abundant resources as raw materials and advantageously employ environmentally-friendly manganese, and therefore have attracted a great deal of attention as a cathode active material capable of substituting LiCoO2. However, these lithium manganese oxides suffer from shortcomings such as low capacity and poor cycle characteristics.
Whereas, lithium/nickel-based oxides including LiNiO2 are inexpensive as compared to the aforementioned cobalt-based oxides and exhibit a high discharge capacity upon charging to 4.3 V. The reversible capacity of doped LiNiO2 approximates about 200 mAh/g which exceeds the capacity of LiCoO2 (about 165 mAh/g). Therefore, despite a slightly lower average discharge voltage and a slightly lower volumetric density, commercial batteries comprising LiNiO2 as the cathode active material exhibit an improved energy density. To this end, a great deal of intensive research is being actively undertaken on the feasibility of applications of such nickel-based cathode active materials for the development of high-capacity batteries. However, the LiNiO2-based cathode active materials suffer from some limitations in practical application thereof, due to the following problems.
First, LiNiO2-based oxides undergo sharp phase transition of the crystal structure with volumetric changes accompanied by repeated charge/discharge cycling, and thereby may suffer from cracking of particles or formation of voids in grain boundaries. Consequently, intercalation/deintercalation of lithium ions may be hindered to increase the polarization resistance, thereby resulting in deterioration of the charge/discharge performance. In order to prevent such problems, conventional prior arts attempted to prepare a LiNiO2-based oxide by adding an excess of a Li source and reacting reaction components under an oxygen atmosphere. However, the thus-prepared cathode active material, under the charged state, undergoes structural swelling and destabilization due to the repulsive force between oxygen atoms, and suffers from problems of severe deterioration in cycle characteristics due to repeated charge/discharge cycles.
Second, LiNiO2 has shortcomings associated with the evolution of excess gas during storage or cycling. That is, in order to smoothly form the crystal structure, an excess of a Li source is added during manufacturing of the LiNiO2-based oxide, followed by heat treatment. As a result, water-soluble bases including Li2CO3 and LiOH reaction residues remain between primary particles and thereby they decompose or react with electrolytes to thereby produce CO2 gas, upon charging. Further, LiNiO2 particles have an agglomerate secondary particle structure in which primary particles are agglomerated to form secondary particles and consequently a contact area with the electrolyte further increases to result in severe evolution of CO2 gas, which in turn unfortunately leads to the occurrence of battery swelling and deterioration of desirable high-temperature safety.
Third, LiNiO2 suffers from a sharp decrease in the chemical resistance of a surface thereof upon exposure to air and moisture, and the gelation of slurries by polymerization of an N-methyl pyrrolidone/poly(vinylidene fluoride) (NMP-PVDF) slurry due to a high pH value. These properties of LiNiO2 cause severe processing problems during battery production.
Fourth, high-quality LiNiO2 cannot be produced by a simple solid-state reaction as is used in the production of LiCoO2, and LiNiMO2 cathode active materials containing an essential dopant cobalt and further dopants manganese and aluminum are produced by reacting a lithium source such as LiOH.H2O with a mixed transition metal hydroxide under an oxygen or syngas atmosphere (i.e., a CO2-deficient atmosphere), which consequently increases production costs. Further, when an additional step, such as intermediary washing or coating, is included to remove impurities in the production of LiNiO2, this leads to a further increase in production costs.
Many prior arts focus on improving properties of LiNiO2-based cathode active materials and processes to prepare LiNiO2. However, various problems, such as high production costs, swelling due to gas evolution in the fabricated batteries, poor chemical stability, high pH and the like, have not been sufficiently solved. A few examples will be illustrated hereinafter.
U.S. Pat. No. 6,040,090 (T. Sunagawa et al., Sanyo) discloses a wide range of compositions including nickel-based and high-Ni LiMO2, the materials having high crystallinity and being used in Li-ion batteries in ethylene carbonate (EC) containing an electrolyte. Samples were prepared on a small scale, using LiOH.H2O as a lithium source. The samples were prepared in a flow of synthetic air composed of a mixture of oxygen and nitrogen, free of CO2.
U.S. Pat. No. 5,264,201 (J. R. Dahn et al.) discloses a doped LiNiO2 substantially free of lithium hydroxides and lithium carbonates. For this purpose, lithium hydroxide and LiOH.H2O as a lithium source are employed and heat treatment is performed under an oxygen atmosphere free of CO2, additionally with a low content of H2O. An excess of lithium “evaporates”; however, “evaporation” is a lab-scale effect and not an option for large-scale preparation. That is, when applied to a large-scale production process, it becomes difficult to evaporate excess lithium, thereby resulting in problems associated with the formation of lithium hydroxides and lithium carbonates.
U.S. Pat. No. 5,370,948 (M. Hasegawa et al., Matsushita) discloses a process for the production of Mn-doped LiNi1−xMnxO2 (x<0.45), wherein the manganese source is manganese nitrate, and the lithium source is either lithium hydroxide or lithium nitrate.
U.S. Pat. No. 5,393,622 (Y. Nitta et al., Matsushita) discloses a process to prepare LiNi1−xMnxO2 by a two-step heating, involving pre-drying, cooking and the final heating. The final heating is done in an oxidizing gas such as air or oxygen. This patent focuses on oxygen. The disclosed method uses a very low temperature of 550 to 650° C. for cooking, and less than 800° C. for sintering. At higher temperatures, samples deteriorate dramatically. Excess lithium is used such that the final samples contain a large amount of water-soluble base (i.e., lithium compounds). According to the research performed by the inventors of the present invention, the observed deterioration is attributable to the presence of lithium salts as impurities which melt at about 700 to about 800° C., thereby detaching the crystallites.
WO 9940029 A1 (M. Benz et al., H. C. Stack) describes a complicated preparation method very different from that disclosed in the present invention. This preparation method involves the use of lithium nitrates and lithium hydroxides and recovering the evolved noxious gasses. The sintering temperature never exceeds 800° C. and typically is far lower.
U.S. Pat. No. 4,980,080 (Lecerf, SAFT) describes a process to prepare LiNiO2-based cathodes from lithium hydroxides and metal oxides at temperatures below 800° C.
In prior arts including the above, LiNiO2-based cathode active materials are generally prepared by high cost processes, in a specific reaction atmosphere, especially in a flow of synthetic gas such as oxygen or synthetic air, free of CO2, and using LiOH.H2O, Li nitrate, Li acetate, etc., but not the inexpensive, easily manageable Li2CO3. Furthermore, the final cathode active materials have a high content of soluble bases, originating from carbonate impurities present in the precursors, which remain in the final cathode because of the thermodynamic limitation. Further, the crystal structure of the final cathode active materials per se is basically unstable even when the final cathode active materials are substantially free of soluble bases. Consequently, upon exposure to air containing moisture or carbon dioxide during storage of the active materials, lithium is released to surfaces from the crystal structure and reacts with air to thereby result in continuous formation of soluble bases.
Meanwhile, Japanese Unexamined Patent Publication Nos. 2004-281253, 2005-150057 and 2005-310744 disclose oxides having a composition formula of LiaMnxNiyMzO2 (M=Co or Al, 1≦a≦1.2, 0≦x≦0.65, 0.35≦y≦1, 0≦z≦0.65, and x+y+z=1). These inventions provide a method of preparing the oxide involving mixing each transition metal precursor with a lithium compound, grinding, drying and sintering the mixture, and re-grinding the sintered composite oxide by ball milling, followed by heat treatment. In addition, working examples disclosed in the above prior art employ substantially only LiOH as a lithium source. Further, it was found through various experiments conducted by the inventors of the present invention that the aforesaid prior art composite oxide suffers from significant problems associated with a high-temperature safety, due to production of large amounts of impurities such as Li2CO3.
Alternatively, encapsulation of high Ni—LiNiO2 by SiOx protective coating has been proposed (H. Omanda, T. Brousse, C. Marhic, and D. M. Schleich, J. Electrochem. Soc. 2004, 151, A922.), but the resulting electrochemical properties are very poor. In this connection, the inventors of the present invention have investigated the encapsulation by LiPO3 glass. Even where a complete coverage of the particle is accomplished, a significant improvement of air-stability could not be made and electrochemical properties were poor.
Therefore, there is a strong need for the development of a LiNiO2-based cathode active material that can be produced at a low cost from inexpensive precursors, and which show improved properties such as low swelling when applied to commercial lithium secondary batteries, improved chemical stability and improved structural safety, and high capacity.